1. Field of Application
The present invention relates to an electric generator control apparatus for controlling the generated output of an electric generator of a motor vehicle.
2. Description of Prior Art
A motor vehicle is equipped with an electric generator, typically constituted as a 3-phase alternator that is driven by the vehicle engine, in combination with a rectifier circuit, to produce a DC output voltage for charging the vehicle battery and supplying an electrical load. The level of generator output voltage is controlled by varying a duty ratio of successive on/off switching (performed by a control apparatus, i.e., regulator) of a drive voltage applied to the field winding of the alternator rotor of the electric generator (referred to in the following simply as the field winding of the electric generator).
In recent years proposals have been made for utilizing an induced current that flows through the field winding at each switch-off of the drive voltage, as a regenerative current to be supplied to the battery. Conventionally, that induced field current has been dissipated by flowing back into the field winding through a diode, with such a type of operation being referred to in the following as a back current mode.
Japanese patent publication No. 62-203599 (pages 2–4, FIGS. 1–3), referred to in the following as reference document 1, describes an example of utilizing that induced field current as a regenerative current. Reference document 1 describes a vehicle electric generator control apparatus having an electric bridge circuit with two opposing arms formed of diodes and the remaining two opposing arms formed of power transistors (where the term “opposing arms” of a bridge circuit, as used herein, signifies two arms which do not have a common junction), and with two opposing junctions of the arms being connected across the battery and the remaining opposing junctions being connected across the field winding. Control of the average voltage applied to the field winding is performed by varying the duty ratio of synchronized on/off switching of the power transistors, and the current induced in the field winding at each switch-off passes through the diodes into the battery, as a regenerative current. Such an on/off switching control arrangement is sometimes referred to as a “transistor chopper” system.
However with the apparatus of reference document 1, the supplying of the regenerative current to the battery is performed irrespective of the generating condition of the electric generator. As a result, when the electric generator has a high level of available generating capacity (i.e., the vehicle engine is running at high speed) and the load on the electric generator is low, so that the battery has attained a substantially fully charged state, the battery will be incapable of absorbing the regenerative current as a charging current. In that condition, each time switch-off of the power transistors occurs, electrical noise in the form of a voltage spike will be produced at the battery supply terminal (generally referred to as the B terminal), as a sudden flow of regenerative current augments the output current from the electric generator. These noise spikes are transferred through wiring that is connected from the battery to various parts of the vehicle electrical system. Hence, such noise spikes can damage or cause malfunctioning of the electric generator or other electronic equipment of the vehicle.
As a countermeasure against this problem, it has been proposed in the prior art to configure such an electric generator control apparatus to be capable of being changed over between:                (a) a control mode in which the induced current of the field winding, at each switch-off of the power transistors of the bridge circuit, is passed directly back into the field winding (through a diode), i.e., a back current mode, and        (b) a control mode in which the induced current of the field winding, at each switch-off of the power transistors of the bridge circuit, is passed into the battery, as a regenerative current, with such a mode being referred to in the following as a regenerative current mode.        
However the relationship between the average voltage that is applied to the field winding and the duty ratio of the successive on/off switching of the power transistors of the bridge circuit, is found to differ in accordance with whether the back current mode or the regenerative current mode is established. As a result, each time there is a changeover between these two modes, an abrupt change may occur in the average voltage that is applied to the field winding. This will result in corresponding abrupt changes in the output current of the generator and in the level of generator torque (the amount of torque that must be applied by the vehicle engine to drive the rotor of the generator). Such abrupt changes in torque are particularly undesirable, since they can result in unstable running of the vehicle engine when it is operating at low speed, e.g., when idling.
With present-day types of electric generator control system for vehicles, instead of simply determining the duty ratio for on/off switching of the power transistors based on an error between the output voltage of the electric generator and a target value (to thereby adjust the field current in accordance with changes in generator load), appropriate values for the duty ratio are selectively read out (i.e., based on the aforementioned error) from a memory. These values are predetermined such as to prevent abrupt changes in the average voltage applied to the field winding, with conventional (i.e., back current mode) control, and so prevent sudden changes in the level of output current of the electric generator and abrupt variations in the generator torque. However such abrupt changes will still occur at each changeover between the back current mode and the regenerative current mode, as described above.
Due to the above-described disadvantages of electrical noise and sudden changes in generator torque, an electric generator control apparatus utilizing a “transistor chopper” type of bridge circuit, enabling regenerative operation as described above, has not come into widespread use, in spite of the advantage of increased efficiency of electrical generation that can be achieved through regenerative operation.